The subject invention pertains to an absorbent polymer having reduced levels of unassociated dust, to an attrition-resistant absorbent polymer and to a process for the preparation of such polymers, and to a process for determining the amount of unassociated matter having a diameter less than or equal to a predetermined size.
In the gel polymerization of water-swellable polymers, monomers are polymerized in aqueous solution. Certain additives, such as crosslinking agents, may be incorporated into the monomer mixture. The product of the polymerization process is typically dried and subjected to mechanical means of particle size reduction and classification including chopping, grinding, and sieving. During such particle size reduction, ultrafine particles or dust, i.e., particles having a mean diameter less than about 10 microns, are unavoidably created.
Japanese Patent Application 6200745-A discloses a water-swellable resin composition having low levels of dust prepared bytreating the polymer with an aqueous solution of a salt and/or hydroxide of a polyvalent metal, the salt and/or hydroxide of the polyvalent metal being provided in the amount of 0.01 to 10 weight percent, based on the weight of the dried polymer.
European Patent Application 103,058 discloses a water-absorbent material consisting of an intimate mixture of 5 to 95 weight percent particulate hydrogel material and 95 to 5 weight percent filler material, which filler material has been treated with up to 1 weight percent of a surfactant. The disclosed materials are said to have reduced dusting tendencies. The disclosed technology, however, would not lead to decreased dusting or the decreased attrition at the site of the manufacture of the hydrogel or during subsequent transport and handling of the hydrogel.
U.S. Pat. No. 4,080,358 discloses a low dusting, free-flowing composition comprising a mixture of a water-soluble acrylamide polymer primarily of a particle size less than 0.5 mm, from 0.1 to 0.5 weight percent based on the weight of the acrylamide of an antidusting agent, e.g., a propylene oxide adduct of glycerol of an average molecular weight between 500 and 2000, and from 1 to 80 weight percent of a highly silicious free-flow aid of submicron to 10 micron particle size.
U.S. Pat. No. 4,734,478 discloses a water-swellable polymer wherein the molecular chains near the surface of the particles are crosslinked. The polymer is prepared by mixing 100 parts water-swellable powder with from 0.001 to 10 parts polyhydric alcohol, and heating the mixture to 90.degree. C. or more to react them. The surface crosslinked particles are said to have no decrease in flowability.
Industry would find great advantage in an aqueous fluid absorbent material which does not dust, e.g., which comprises no more than 2.5 ppm unassociated particles less than 10 microns in diameter. Industry would further find advantage in an attrition-resistant aqueous fluid absorbent material, e.g., which comprises no more than 35 ppm unassociated dust upon roll-milling for 20 minutes, and/or which comprises no more than 15 ppm unassociated dust upon roll-milling for 10 minutes. Industrywould further find advantage in a process for reducing the level of particles less than 10 microns in diameter in a given composition by at least 90 percent. Industry would further find advantage in a process for ascertaining the amount of unassociated dust in a water-swellable polymer composition.